Nagy R Chelsea, Porder Stephen, Brando Paulo, Davidson Eric A, Figueira Adelaine Michela E Silva, Neill Christopher, Riskin Shelby, Trumbore Susan
Ecology and Evolutionary Biology Brown University Providence RI USA.
Ecosystems Center Marine Biological Laboratory Woods Hole MA USA.
J Geophys Res Biogeosci. 2018 Jan;123(1):18-31. doi: 10.1002/2017JG004269. Epub 2018 Jan 5.
Climate and land use models predict that tropical deforestation and conversion to cropland will produce a large flux of soil carbon (C) to the atmosphere from accelerated decomposition of soil organic matter (SOM). However, the C flux from the deep tropical soils on which most intensive crop agriculture is now expanding remains poorly constrained. To quantify the effect of intensive agriculture on tropical soil C, we compared C stocks, radiocarbon, and stable C isotopes to 2 m depth from forests and soybean cropland created from former pasture in Mato Grosso, Brazil. We hypothesized that soil disturbance, higher soil temperatures (+2°C), and lower OM inputs from soybeans would increase soil C turnover and deplete C stocks relative to nearby forest soils. However, we found reduced C concentrations and stocks only in surface soils (0-10 cm) of soybean cropland compared with forests, and these differences could be explained by soil mixing during plowing. The amount and ΔC of respired CO to 50 cm depth were significantly lower from soybean soils, yet CO production at 2 m deep was low in both forest and soybean soils. Mean surface soil δC decreased by 0.5‰ between 2009 and 2013 in soybean cropland, suggesting low OM inputs from soybeans. Together these findings suggest the following: (1) soil C is relatively resistant to changes in land use and (2) conversion to cropland caused a small, measurable reduction in the fast-cycling C pool through reduced OM inputs, mobilization of older C from soil mixing, and/or destabilization of SOM in surface soils.
气候和土地利用模型预测，热带地区的森林砍伐和向农田的转变将因土壤有机质（SOM）加速分解而向大气中释放大量土壤碳（C）。然而，目前大多数集约化作物农业正在扩张的热带深层土壤中的碳通量仍受到的限制较少。为了量化集约化农业对热带土壤碳的影响，我们比较了巴西马托格罗索州由前牧场改造而来的森林和大豆农田2米深度内的碳储量、放射性碳和稳定碳同位素。我们假设，与附近的森林土壤相比，土壤扰动、较高的土壤温度（+2°C）以及大豆较低的有机物质输入会增加土壤碳周转并消耗碳储量。然而，我们发现，与森林相比，大豆农田仅表层土壤（0 - 10厘米）中的碳浓度和储量有所降低，这些差异可以用耕作过程中的土壤混合来解释。大豆土壤中50厘米深度内呼吸产生的二氧化碳量和ΔC显著较低，但森林和大豆土壤在2米深处的二氧化碳产生量都很低。2009年至2013年期间，大豆农田表层土壤的平均δC下降了0.5‰，这表明大豆的有机物质输入较低。这些研究结果共同表明：（1）土壤碳对土地利用变化具有相对抗性；（2）向农田的转变通过减少有机物质输入、土壤混合中旧碳的移动和/或表层土壤中土壤有机质的不稳定，导致快速循环碳库出现了微小但可测量的减少。
